With the increasing requirements of electric equipment for the capacity of lithium-ion batteries, people have higher and higher expectations for the improvement of energy density of lithium-ion batteries. In particular, smart phones, tablets, laptops and other portable devices put forward higher requirements for lithium-ion batteries with small volume and long standby time. Similarly, in other electric equipment, such as energy storage equipment, electric tools, electric vehicles, etc., lithium-ion batteries with lighter weight, smaller volume, higher output voltage and power density are also being developed. Therefore, the development of lithium-ion batteries with high energy density is an important R & D direction of the lithium battery industry.
1、 Development background of high voltage lithium ion battery
In order to design lithium-ion batteries with high energy density, in addition to continuously optimizing its space utilization, improving the compaction density and gram capacity of battery anode and cathode materials, and using high conductive carbon nano and polymer adhesives to improve the content of positive and negative active substances, increasing the working voltage of lithium ion batteries is also one of the important ways to increase the energy density of batteries.
The cut-off voltage of lithium-ion battery is gradually changing from the original 4.2V to 4.35v, 4.4V, 4.45v, 4.5V and 5V. Among them, 5V nickel manganese lithium-ion battery has excellent characteristics such as high energy density and high power, which will be one of the important directions for the development of new energy vehicles and energy storage in the future. With the continuous development of power R & D technology, lithium-ion batteries with higher voltage and higher energy density will gradually go out of the laboratory to serve consumers in the future.
2、 Application status of high voltage lithium ion battery
Generally speaking, high-voltage lithium-ion batteries refer to batteries with a single charging cut-off voltage higher than 4.2V, such as lithium-ion batteries used in mobile phones. The cut-off voltage develops from 4.2V to 4.3v, 4.35v, and then to 4.4V (Xiaomi mobile phones, Huawei mobile phones, etc.). At present, 4.35v and 4.4V lithium-ion batteries have been mature in the market, and 4.45v and 4.5V have also begun to be favored by the market and will gradually develop and mature.
At present, battery manufacturers of mobile phones and other digital electronic products at home and abroad are moving towards high-voltage lithium-ion batteries. Lithium ion batteries with high voltage and high energy density will have greater market space in high-end mobile phones and portable electronic devices. Cathode materials and electrolyte are the key materials to improve the high voltage of lithium-ion batteries. Among them, the use of modified high voltage lithium cobalt oxide and high voltage ternary materials will be more mature and common.
With the increase of voltage, some safety performance of high-voltage lithium-ion batteries will be reduced in the process of use, so they have not been used in batches in power vehicles. At present, the battery cathode materials used in power vehicles are mainly ternary materials and lithium iron phosphate. In order to improve the energy density and meet the demand, high nickel cathode materials such as 811ncm and NCA, high-capacity silicon carbon cathode or improving the utilization of battery space are generally selected to improve its energy density and endurance.
3、 Progress of main materials and processes of high voltage lithium ion batteries
The performance of high voltage lithium-ion battery is mainly determined by the structure and properties of active materials and electrolyte, in which the cathode material is the most key core material, and the matching effect of electrolyte is also very important. The following mainly analyzes the current research and application status of high voltage cathode materials.
1. Research status of high pressure lithium cobalt oxide materials
At present, the most widely studied and applied high voltage cathode material is lithium cobalt oxide, which has two-dimensional layered structure. Structure, α- Nafeo2 type is more suitable for lithium ion embedding and stripping. The theoretical energy density of lithium cobalt oxide is 274mah / g. it has the advantages of simple production process and stable electrochemical properties, so it has a high market share. In practical application, only part of lithium ions can be reversibly embedded and removed, and its actual energy density is about 167mah / g (working voltage is 4.35v). Increasing its working voltage can significantly improve its energy density. For example, increasing its working voltage from 4.2V to 4.35v can increase its energy density by about 16%.
However, under high voltage, lithium ions are embedded and removed from the material for many times, which will change the structure of lithium cobaltate from cubic system to monoclinic system. At this time, lithium cobaltate material no longer has the ability to embed and remove lithium ions. At the same time, the particles of positive material loosen and fall off from the collector, resulting in increased internal resistance and poor electrochemical performance of the battery.
At present, the modification of lithium cobalt oxide cathode materials mainly improves the crystal structure stability and interface stability from the two aspects of doping and coating.
At present, lithium cobalt oxide high-voltage materials have been used in high-energy density batteries in batches. For example, high-end mobile phone battery manufacturers have higher and higher requirements for battery performance, which is mainly reflected in higher requirements for energy density. For example, the energy density of 4.35v mobile phone battery with carbon as negative electrode is about 660wh / L, and the energy density of 4.4V mobile phone battery has reached about 740wh / L, This requires the cathode material to have higher compaction density, higher void play, and better stability of the material structure under high voltage and high voltage. However, lithium cobalt oxide electrode material has some disadvantages, such as lack of cobalt resources and high price. In addition, cobalt ion has certain toxicity, which limits its wide application in power batteries.
2. Research status of ternary materials
In order to reduce the amount of cobalt and improve the safety performance of the battery, researchers began to focus on the research of layered ternary high voltage materials (linixcoymn1-x-yo2 or linixcoyal1-x-yo2). Among these ternary materials, nickel (Ni) plays a role in providing capacity, cobalt (CO) can reduce the mixed discharge of lithium (LI) and Ni, and manganese (MN) or aluminum (AL) can improve the structural stability of layered materials, so as to improve the safety performance of batteries. This type of battery is mainly used for general conventional digital batteries, such as rechargeable bank and business standby battery. It is regarded as a substitute for lithium cobalt oxide to improve the price competitiveness of the battery. The most common is that the ratio of nickel, cobalt and manganese is 5 ∶ 2 ∶ 3.
In terms of power vehicles, many manufacturers are trying to improve the energy density mainly by increasing the working voltage of single lithium-ion batteries and increasing the nickel content in ternary materials. However, at present, the industry is still in the development stage and there are no batch products. This is mainly because at present, the power battery must first meet the high safety, consistency, low cost and long service life of the battery, and the improvement of capacity is not the primary problem.
The main problem of ternary materials is that with the increase of nickel content, the alkalinity of materials becomes stronger, and the requirements for battery manufacturing technology and environment are higher and higher; At the same time, the thermal stability of the material decreases, and oxygen will be released during the cycle, resulting in the deterioration of the structural stability of the material; In the charged state, nickel has strong oxidation, which also puts forward higher requirements for the matching of electrolyte. Therefore, ternary electrode materials have high limitations in popularization and application.
3. Research status of manganese based cathode materials
Lithium manganate is a typical Spinel Cathode material. The theoretical energy density reported in the literature is 148mah / g, which is lower than lithium cobalt oxide and ternary materials. It has the characteristics of low price, high thermal stability, environmental friendliness and easy preparation. It is expected to be widely used in energy storage batteries and power batteries.
In terms of power batteries, lithium manganate is not widely used in China compared with ternary materials and lithium iron phosphate, mainly due to its low energy density and poor cycle life, resulting in the problems of short battery life and low service life. The cycle performance of lithium manganate, especially the cycle performance at high temperature (55 ℃), has been criticized. Its main influencing factors are divided into three aspects:
① Dissolution of Mn3 + on the surface. At present, the lithium salt used in the conventional electrolyte is lithium hexafluorophosphate (LiPF6), and the electrolyte itself contains a certain amount of hydrofluoric acid (HF) impurities. Trace water in the battery system will lead to the decomposition of LiPF6 to produce HF. The existence of HF will erode lithium manganate (LiMn2O4) and lead to the disproportionation and dissolution of Mn3 +, 2mn3 + (solid phase) → Mn4 + (solid phase) + Mn2 + (liquid phase). At the end of discharge and under the condition of high rate discharge, the content of Mn3 + on the material surface is higher than that in the bulk phase, which intensifies the dissolution of Mn3 + on the material surface.
② Ginger Taylor effect. Li1 generated on the material surface during battery discharge, especially in the case of over discharge+ δ［ Mn2] O4 is thermodynamically unstable. At the same time, the material structure changes from cubic phase to tetragonal phase, and the original structure is destroyed, so the cyclic performance of the material becomes worse.
③ High oxidation of Mn4 +. Li1 with high lithium removal at the end of charging or in the case of overcharge+ δ［ Mn4 + in Mn2] O4 material has strong
It can oxidize and decompose organic electrolyte and deteriorate the cycle performance of the battery. At present, the energy density of most lithium manganate batteries is less than 100mah / g, the normal temperature cycle can only reach 400 ~ 500 times, and the high temperature cycle can only reach 100 ~ 200 times, which can not meet the demand of mass production. But in fact, the battery system of Nissan LEAF, which accounts for nearly 20% of global electric vehicle sales, is the lithium manganate battery, with a range of about 200km.
Although the performance of lithium manganate battery is restricted by the structure of the material itself, as long as the disadvantages of low energy density and poor cycle performance are solved, it still has a very broad application space in the field of power battery in the future.
In order to improve the energy density and cycle performance of lithium manganate electrode materials, some researchers improve the voltage of cathode materials by doping modification, such as LiMxMn2-xO4 [(M = chromium (CR), iron (FE), Co, Ni, copper (Cu)] 5V high voltage cathode materials, among which the nickel manganese high voltage material LiNi0.5Mn1.5O4 is the most widely studied. The specific discharge capacity of Ni Mn high voltage material is up to 130mah / g, the platform can reach about 4.7V, the energy density is higher than that of lithium cobaltate under conventional working voltage, and there is basically no ginger Taylor effect of Mn3 +.
When the working voltage is increased to about 5V, compared with traditional lithium cobalt oxide, lithium manganate, ternary and lithium iron, nickel manganese high voltage material has the advantages of high gram capacity, high discharge platform, high safety performance and rate performance. It has great advantages in the matching of battery pack, but its high-temperature performance and cycling need to be improved. From the current application, it is still in the small batch production stage of steel shell battery, and there is still a long way to go for the doping modification and surface coating of nickel manganese high voltage materials.
4. Research status of high voltage electrolyte
Although high voltage lithium ion batteries have made great contributions to improving the energy density of batteries, there are also many problems. With the increase of energy density, generally, the compaction density of positive and negative electrodes is relatively large, the wettability of electrolyte becomes worse, and the liquid holding capacity decreases. Low liquid retention will lead to poor cycling and storage performance of the battery. In recent years, with the continuous emergence and application of high-voltage cathode materials, conventional carbonate and lithium hexafluorophosphate systems will decompose in batteries with voltage above 4.5V, resulting in poor cycle performance and poor high-temperature performance, which can not fully meet the requirements of high-voltage lithium-ion batteries. Therefore, it is of great significance to study the electrolyte system matching these high voltage cathode materials.
In view of the poor wettability of electrolyte caused by high-voltage solid density, electrolyte design continues to screen solvents with high oxidation potential and low viscosity to meet the performance requirements of high-voltage solid battery. In addition, additives or fluorinated solvents that can improve the wettability of electrolyte are also used to improve, and the effect is also obvious.
